The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

Four glass boxes hover over 1,000 feet in the air to give visitors an unparalleled view of Chicago.

A peculiar mansion built by the troubled heir to the Winchester Rifle Company fortune.

A "gravitational anomaly" located in the redwood forests just outside of Santa Cruz, California.

FDR hoped to find a cure for polio in this Georgia town's warm springs.

A hidden subterranean installation offers a view of the sky.

An elaborate gift from Claus Spreckels is now a piece of San Francisco history.

A masterpiece in Golden Gate Park by virtue of the artist’s debt.

Architecture lovers won’t stop touching the National Gallery's 19.5 degree marble prow.

A muscular Art Deco monument represents the struggle between regulators and unbridled markets.

This mysterious structure by the Washington Monument is a flood barrier designed to protect the White House against rising waters.

Perhaps the most scandalous statue of America's first president.

A much-maligned monument to teetotalism.

A heroic pigeon that, through a barrage of gunfire, delivered a message that saved over 100 lives in World War I.

The "lovely red Vega" of the legendary record-settling pilot.

The remains of Martha, the last known passenger pigeon, serve as a tool to educate about conservation.

Light bulbs, scrap wood, and tinfoil comprise this homemade throne of the gods.

The actual model from the original "Star Trek" series is now on display at the National Air and Space Museum.

The most decorated dog of World War I is preserved in the Smithsonian.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

While most wind tunnels test scale models, the "Cave of Winds" was large enough for actual airplanes.

This isn't your average roadway—it's actually a National Park and a transportation pioneer.